Corona devices with pin electrodes are well known. See, for example, U.S. Pat. No. 3,744,898; Research Disclosure No. 184, Aug. 1979, p. 428 ( 18444) and European Patent Application 0 274 895 Al (comb electrode). These devices are used, for example, for charging a photoconductive element in an electro-photographic dsevice, or for creating a field required tro transfer a powder image from the photoconductive element to a receiving material. On connection to a high voltage each pin electrode generates an ion cloud which extends from the pin electrode to a counter electrode. A material to be charged, e.g. a photoconductive element, is situated between the pin electrodes and the counter electrode.
A disadvantage of such corona devices is that the ion clouds repel one another so that a non-uniform charge pattern may occur on the material to be charged.
A pin electrode corona device having an array of pins and auxiliary electrode is disclosed in Netherland Application NL-A-294832. This application describes a corona device in which a strip-shaped auxiliary electrode is disposed on either side of and parallel to an array of pin electrodes. Charge uniformity is improved by the use of such auxiliary electrodes in the vicinity of the pin electrodes. This use, however, in turn has the disadvantage that the charging speed is reduced. Accordingly, it is an object of the present invention to provide improved auxiliary electrodes which overcome this disadvantage.